Bears defensive coordinator Vic Fangio is off to run the Broncos … now what?

Presented with the possibility last week that defensive coordinator Vic Fangio could leave the Bears — a hypothetical that came true Wednesday, when Fangio agreed to become the Broncos’ next coach — Matt Nagy vowed that he was prepared.

“That’s something that when the time comes, if you have to deal with that,” Nagy said, “you have a plan ready to go.”

Now we’ll see how detailed Nagy’s plan was.

Within hours of Fangio, 60, agreeing to take the job in Denver, the Bears’ first choice to replace him came off the board. Former Jets coach Todd Bowles, who played for Nagy’s dad at Elizabeth (New Jersey) High School, will join the Buccaneers as their defensive coordinator.

Bears defensive coordinator Vic Fangio walks onto the field before a game against the Rams.| Nam Y. Huh/AP photo

Bowles, a source said, decided to keep his word to new Bucs coach Bruce Arians, whom he’d worked for in Arizona and played for at Temple, despite the temptation to join a defense that was ranked No. 1 by most metrics. Bowles considers Arians a father figure. Nagy’s close friendship wasn’t enough for Bowles to go back on his pledge to his mentor.

Nagy has never hired a defensive coach. When he joined the Bears, he opted — wisely, in retrospect — to keep Fangio’s staff intact.

Fangio’s skill in building the defense over four years makes his vacancy — the Bears’ coordinator opening — the most coveted in the NFL. When Fangio joined the Bears in 2015, he inherited a defense that, in Marc Trestman’s last year, allowed the second-most points and third-most yards in the NFL. He departs with his defense allowing the fewest points and third-fewest yards.

Broncos general manager John Elway noticed and acted fast. He interviewed Fangio on Monday and chose him over finalist Mike Munchak after a day of deliberation.

All but one Bears defensive starter — free-agent safety Adrian Amos — is under contract for next season. The defense boasts four Pro Bowl players: outside linebacker Khalil Mack, defensive lineman Akiem Hicks, safety Eddie Jackson and cornerback Kyle Fuller. Three of them — all but Hicks — were named first-team All-Pros.

That skill level makes the job opening so attractive — as does the potential of autonomy. In his first season, Nagy focused on building up the Bears’ offense and developing quarterback Mitch Trubisky. That offensive bent figures to continue into Nagy’s second season.

Were Nagy to opt for continuity — it worked a year ago — he could promote secondary coach Ed Donatell, who’s responsible for developing Jackson and Fuller into dominant players this season. Donatell is the only Bears defensive staffer with coordinator experience. He ran the Packers’ defense from 2000 to 2003 and the Falcons’ from 2004 to 2006. After coaching alongside Fangio with the 49ers and Bears, though, Donatell could be a threat to follow his boss to Denver.

If the Bears look outside Halas Hall, there are no coordinator candidates with direct lines to Nagy — and certainly none as familiar to him as Bowles.

Jack Del Rio, the former Raiders and Jaguars coach, is close with Mack from their days in Oakland. Gregg Williams, the Browns’ interim coach at the end of the year, knows general manager Ryan Pace from their days with the Saints, but he has the blemish of a “Bountygate” suspension. Former Colts coach Chuck Pagano — as well as fired 2018 head coaches Steve Wilks, a former Bears assistant, and Vance Joseph — could be intrigued.

That’s the other attractive part of the Bears’ opening: Coordinators with hopes of returning to the head-coaching ranks could see success with a loaded roster as the fast track back to the top.

Whoever it is, the Bears’ next coordinator will have a hard time being as beloved inside the locker room as Fangio, the “Evil Genius” whose departure players had openly dreaded.

On Wednesday, he did just that, leaving the Bears in an unfamiliar position: victims of their own success.